1. Technical Field
The present invention pertains to full-duplex data communication devices for dial-up 2-wire telephone circuits, and more particularly to such devices for on-line, interactive operation of remote data terminal equipment from a host processor.
2. Background
Selecting dial-up 2-wire telephone circuits versus dedicated or conditioned lines for interactively operating remote data terminal equipment (DTE), such as video display terminals and printers, typically has involved a susbstantial trade-off between cost and performance.
Data communication equipment (DCE) optimized for remote-to-host on-line operation typically use synchronous data communication over dedicated and/or conditioned telephone facilities. That approach necessarily reduces the utility of remote DTE because economics often dictates using less expensive asynchronous DTE and the least expensive, and therefore lowest quality, telephone communication facilities--dial-up 2-wire switched telephone circuits (the DDD network). Compared with more costly data communication links, such circuits have a limited passband (approximately 2800 Hz), and are prone to communication errors due to noise or other disturbances on the telephone circuit.
For example, asynchronous DTE typically operate at symmetrical transmit/receive speeds of 9600 bps. Typical asynchronous DCE used for communication over the DDD network, however, communicate at 1200 bps, which limits the utility of remote asynchronous DTE due to inadequate response time.
Some improvement in effective Host/DTE communication rate can be achieved using different modulation schemes. For full-duplex operation, asynchronous modems typically allocate one-half of the telephone circuit passband to each direction. One technique for improving communication throughput uses split-speed modems that asymmetrically allocate bandwidth to provide a high speed channel (e.g., 2000 Hz bandwidth) and a low speed channel (e.g., 300 Hz bandwidth). Split-speed operation takes account of the fact that remote DTE such as a video display terminal typically can receive data from the Host (i.e., for screen display) at a much higher rate than it will transmit (i.e., as a result of keyboard entry). Split-speed DCE, however, typically operates synchronously and requires a synchronous, split-speed DTE I/O interface, precluding its use with standard asynchronous DTE having symmetrical I/O rates.